It has long been known to pressurise aircraft cabins, so that people can spend time in such aircraft cabins even at relatively high flying altitudes without aids such as oxygen masks, etc. Conventionally, so-called bleed air, which is tapped from a compressor stage of the aircraft engine or engines, is used to pressurise an aircraft cabin. This already pressurised bleed air is decompressed to the required cabin pressure in an aircraft air-conditioning system, cooled and otherwise conditioned before being supplied to the aircraft cabin. The air tapped from an engine increases the fuel consumption of the engine, since the tapped proportion of air is no longer available to the engine for thrust generation, so that the compressor of the engine must be designed larger than is actually required purely for thrust generation.
In order to reduce the requirement for bleed air, it is known to skilled persons in the field of aircraft cabin air-conditioning to clean a certain proportion of the air already present in the pressure cabin in a recirculation air circuit and then feed said proportion back to the pressure cabin, instead of constantly supplying outside air produced from bleed air to the pressure cabin.
In order to save weight in aircraft with pressure cabins, the cabin pressure is set so that it corresponds approximately to the air pressure prevailing at an altitude of 2500 m. In this way the pressure differential acting between the pressure cabin and the ambient atmosphere during flight can be kept small, leading to reduced strength requirements and, as a result, lower weight of the aircraft fuselage structure. However, a disadvantage is that, because of the procedure mentioned, the oxygen partial pressure in the aircraft cabin is significantly lower than at ground level, which can be detrimental to comfort during long flights, especially for older and sick passengers. For example, the oxygen partial pressure at ground level is approximately 200 mbar, whereas it is only 175 mbar in the pressure cabin of a commercial aircraft flying at high altitude. To solve this problem it is proposed in
German patent 196 45 764 describes a device to enrich the fresh air supplied to the pressure cabin of an aircraft with oxygen. A membrane module which separates the oxygen fraction contained in the exterior air is proposed as the device for enriching the fresh air with oxygen. However, this procedure results in increased consumption of bleed air, since only the proportion of oxygen obtained from the (additional) bleed air by means of the oxygen enrichment device is supplied to the pressure cabin, while the remainder of this additional bleed air escapes unused.
A further possible way of increasing the oxygen partial pressure in an aircraft pressure cabin is disclosed in GB 2 397 821 A. In that document, according to one embodiment the recirculation air is passed through a membrane module in order to generate an oxygen-rich and a nitrogen-rich gas stream, the oxygen-rich gas stream being fed back into the aircraft cabin.
It is the object of the invention to increase the oxygen partial pressure in an aircraft pressure cabin without additional consumption of bleed air, and to execute the raising of the oxygen partial pressure in the pressure cabin with greater overall efficiency.